You are correct, you can't reliably judge when to harvest by the hairs. It can give you a rough idea. It is possible you don't see any balls on the sticks is because it is past harvest time, and they have fallen off of wherever you're looking. It is also possible that you have a strain that doesn't produce much resin. That doesn't necessarily mean that it will be weak, but it is more likely. It's also possible that you have a strain that takes a long time to finish, and the balls on sticks are not apparent yet.